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Centrifuges

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Image Source: ProfiLab24
Image result for supernatant and pellet     A centrifuge is an instrument which separates any suspension or floating colloidal particles in the solution into fractions. The resulting fractions are termed as Supernatant and Pellet. Supernatant will contain the solution and Pellet is the particles which are collected at the bottom. A centrifuge works on the principle of centrifugal force, where the centifugal acceleration causes the denser substances and particles to move outward in the radial direction. Denser particles will settle at the bottom forming a pellet and less denser will settle in the center of on top in a solution called as supernatant. Centrifuges are relatively simple devices having stationary and rotary parts. The rotation generated by the electric motor of the centrifuge is transmitted to the rotor harbouring the samples contained within appropriate centrifuge tubes. (A centrifugal force is a force arising from the body's inertia which acts on a body which is moving in a circular path and directed away from the centre.   Some biological samples are sensitive such as some samples are sensitive to heat, for which the centrifuge has to be used in a refrigerating conditions such as 4℃. 


Principle: 

         "The Centrifuge works on the principle of centrifugation, where the acceleration at centripetal force causes denser substances to separate out along the radial direction at the bottom of the tube. In a solution, particles which have higher density will sediment (Pellet) at the bottom and particles that are lighter will float to the top (Supernatant)". "The greater the difference in density of the particles, the faster they move". 

 RCF (Relative Centrifugal Force):

            It is the ration of the centrifugal acceleration at a specified radius and the speed to the standard acceleration of gravity. 

 RCF is defined as f=M⍵2r

       F = Intensity of centrifugal force
      M = Mass of particle
      ⍵ = angular velocity of rotation
      r = distance of migrating particles from central axis of rotation. 

 RCF value for any centrifuge can be calculated from the following equation. 
RCF = 1.18 x r x (rpm)2x10^-5

 1.18x10^-5 =  An empirical factor
r = radius in cm from the centre of the centrifuge shaft to the external tip of centrifuge tube. 
rpm = number of revolutions per minute of the rotor. 




 Types of centrifuge depends on the speed of sedimentation, presence/absence of vacuum, temperature control or refrigeration, volume of sample and capacity of centrifuge tubes and types of biological samples to be used for the centrifugation. 
Types of Centrifuges:

 1) Small Bench Top Centrifuge:

              This type of centrifuges are common in research and clinical labs. (For example, separation of blood/plasma or serum).. It can be used with or without refrigeration. has slow speed up to 4000 to 5000 rpm. 
Image result for eppendorf 5804
Eppendorf 5804 Centrifuge
2) Micro Centrifuge: 

        This type of centrifuge can take small tubes with volume of up to 2mL. can be used with or without refrigeration. can go up to 12,000 rpm. It is commonly used in concentrating protein and DNA/RNA Isolation. 
Image result for micro centrifuge
Eppendorf Mini Centrifuge


3) High Speed Centrifuge:
         This type of centrifuges are mostly used in refrigerated conditions. some centrifuges of this type can reach up to 60,000 to 70,000 rpm. Mostly used for protein precipitation, large intact organelles, cellular debris from tissue homogenization and micro organisms. Especially used in research facilities. It can also be used for the differential separation of nucleus, protein precipitate, mitochrodial, etc. 
Image result for high speed centrifuge
BeckMan High Speed Centrifuge


4) Ultra Centrifuge: 
            This type of centrifuge is mostly used for the detailed biochemistry analysis of sub cellular structures and isolate biochemicals. Speed can reach up to 90,000rpm to 100,000 rpm. used in refrigerated and evacuated conditions. 
Image result for ultracentrifuge
BackMan, Ultra Centrifuge






Types of Rotors:
        
              There are mainly two types of rotors used in centrifuges, "Fixed Angle Rotor" and "Swinging Bucket Rotor".

Fixed Angle Rotor: Centrifuge tubes are held at fixed angle of between 14' to 40' to vertical axis of rotation. These rotors are useful for pelleting. "Start of centrifugation particles are driven outward horizontally but strike side of the tube so that sediment pack against the side and bottom of the tube, with the surface of the sediment parallel to the shaft of centrifuge. 
Image result for fixed angle rotor
Hettich® centrifuge

Swinging Bucket Rotor:
        Sample tubes are loaded into the individual buckets that hang vertically while the rotor is at rest. When the rotor begins to rotate the buckets swing out to a horizontal position. This centrifuge is useful when sample are to be resolved in density gradients. 
Related image
Eppendorf Swinging Bucket Rotor


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